The invention is related to a video transmission system comprising a transmitter with a video source for providing an encoded video signal, the transmitter comprising transmit means for transmitting the encoded video signal via a packet switched network to a receiver.
The invention is also related to a transmitter for use in such a transmission system and to a video transmission method.
A transmission system as described above is known from published European patent application No. 517 273 A2.
In order to transmit video signals in digital form, the video signal is encoded using a video encoder. Because the pictures in a video signal can have different properties, the bitrate required for encoding a video signal with a predetermined quality is not constant, but will be variable. The bitrate of an encoded video signal depends on the amount of detail in a picture and the amount of motion present in a sequence of pictures.
In view of this variable bitrate it is expected that in the near future packet switched networks will become important for the transport of variable bitrate encoded video signals. These packet switched networks will likely operate according to the ATM standard. Packet switched networks are in particular suitable for transporting user signals having a variable bitrate, because they do not build up a circuit for the complete connection, but they only pass packets from a source to a destination via a path determined at call set up. If, for instance, temporarily no packets are to be transmitted, the resources required for said transmission can be used for transmitting packets from other connections.
In order to avoid congestion in the packet switched network, at call set up the grade of service to be guaranteed by the network is defined in a so-called traffic contract negotiated between the transmitter and the packet switched network. This traffic contract enables the network to allocate resources on a statistical basis, and it poses to the transmitter the bounds within which it can operate. Elements of the connection to be negotiated between the transmitter and the network are e.g. the service categories as CBR (Constant Bit Rate), VBR (Variable Bit Rate), ABR (Available Bit Rate) and UBR (Unspecified Bit Rate ). Grade of service (GoS) parameters are e.g. CLR (Cell Loss Ratio), maxCTD (maximum Cell Transfer Delay) and peak-peakCDV (Cell Delay Variation). A problem with commonly used video encoders is that the required traffic parameters are not always available in advance. The consequence is that the grade of service requested from the network is too low, making it impossible to transmit the encoded video signal, or that the grade of service is higher than required, leading to unnecessary high cost.
In the transmission system according to the above mentioned patent application the video encoder is arranged for reducing the bitrate of the encoded video signals if this bitrate exceeds the grade of service requested from the network. This reduction of the bitrate leads to a decreased quality of the encoded pictures. The fact that sometimes the requested grade of service is higher than required, and consequently the unnecessarily high costs incurred, is accepted.